Method for the production of highly pure nitric acid

ABSTRACT

In a new and improved process for preparing high-purity nitric acid by distillation or rectification of nitric acid, crude nitric acid or mixtures thereof in a column, 85-95% strength by weight nitric acid, crude nitric acid or a mixture thereof is distilled at from 70 to 110° C. and a pressure of from 0.1 to 2 bar.

The present invention relates to a process for preparing high-puritynitric acid by distillation or rectification of nitric acid, crudenitric acid or mixtures thereof in a column, wherein from 85 to 95%strength by weight nitric acid, crude nitric acid or a mixture thereofis distilled at from 70 to 110° C. and a pressure of from 0.1 to 2 bar.

SU-A-11 01 249 discloses a process for preparing high-purity nitric acidfrom 60% strength crude nitric acid by means of a complicated multipledistillation.

This process has the disadvantage that cations such as calcium,magnesium, sodium and zinc occur as impurities in concentrations of morethan 1 ppm.

It is an object of the present invention to remedy the abovementioneddisadvantages.

We have found that this object is achieved by a new and improved processfor preparing high-purity nitric acid by distillation or rectificationof nitric acid, crude nitric acid or mixtures thereof in a column,wherein from 85 to 95% strength by weight nitric acid, crude nitric acidor a mixture thereof is distilled at from 70 to 110° C. and a pressureof from 0.1 to 2 bar.

The process of the present invention can be carried out as follows:

The high-purity nitric acid can be obtained by batchwise or preferablycontinuous distillation, preferably rectification, of from 85 to 95%strength by weight nitric acid, crude nitric acid or mixtures thereof atfrom 70 to 115° C., preferably from 75 to 100° C., particularlypreferably from 80 to 90° C., at atmospheric pressure or under slightlysuperatmospheric or subatmospheric pressure, i.e. in the range from 0.1to 2 bar, preferably from 0.5 to 1.5 bar, particularly preferably atatmospheric pressure. The temperature at the bottom of the distillationcolumn is generally from 100 to 140° C., preferably from 110 to 130° C.,particularly preferably from 115 to 125° C.

In this process, from 85 to 95% strength by weight nitric acid, crudenitric acid or a mixture thereof can be fed directly into the bottom ofthe column or can be fed at any point on the column or, preferably,highly concentrated crude nitric acid is used and a diluent is fed in atany point on the column so that the concentration of the nitric acid tobe purified is from 85 to 95% by weight after this addition. This 85-95%strength by weight nitric acid, crude nitric acid or mixture thereofsubsequently passes through from one to five, preferably from one tothree, particularly preferably from one to two, theoretical plates. Ingeneral, the feed stream can be at ambient temperature, but it isadvantageous to heat the feed stream to, as a rule, from 40 to 90° C.,preferably to from 70 to 90° C., more preferably to from 75 to 85° C.and particularly preferably to from 78 to 82° C.

Highly concentrated nitric acid, crude nitric acid and mixtures thereofcontain from 95 to 100% by weight, preferably from 96 to 99% by weight,particularly preferably from 97 to 98% by weight, of nitric acid and aregenerally obtainable by customary methods, for example by reaction ofNO_(x) and water in an autoclave or from the reaction of NO_(x) andwater and concentration by means of azeotropic distillation afteraddition of sulfuric acid.

The high-purity nitric acid produced according to the present inventiongenerally contains from 0.005 to 0.1 [mg/kg] of sulfate, preferably from0.01 to 0.09 [mg/kg] of sulfate, particularly preferably from 0.02 to0.08 [mg/kg] of sulfate, generally from 0.005 to 0.1 [mg/kg] ofchloride, preferably from 0.01 to 0.09 [mg/kg] of chloride, particularlypreferably from 0.02 to 0.08 [mg/kg] of chloride, and from 0.05 to 1[ppb] of sodium, preferably from 0.07 to 0.8 [ppb] of sodium,particularly preferably from 0.09 to 0.5 [ppb] of sodium, in particularfrom 0.1 to 0.3 [ppb] of sodium, and in addition impurities inconcentrations of 0.1 ppb by weight (ppb=parts per billion) or less percation (with the exception of H⁺). Thus, the chromium content isgenerally from 0.001 to 0.09 [ppb], preferably from 0.005 to 0.07 [ppb],particularly preferably from 0.007 to 0.05 [ppb]. The calcium content isgenerally from 0.005 to 0.1 [ppb], preferably from 0.01 to 0.09 [ppb],particularly preferably from 0.015 to 0.07 [ppb]. The potassium contentis generally from 0.005 to 0.1 [ppb], preferably from 0.01 to 0.09[ppb], particularly preferably from 0.015 to 0.07 [ppb]. The ironcontent is generally from 0.005 to 0.1 [ppb], preferably from 0.01 to0.09 [ppb], particularly preferably from 0.015 to 0.07 [ppb]. Themagnesium content is generally from 0.001 to 0.09 [ppb], preferably from0.005 to 0.07 [ppb], particularly preferably from 0.007 to 0.05 [ppb].The zinc content is generally from 0.005 to 0.1 [ppb], preferably from0.01 to 0.09 [ppb], particularly preferably from 0.015 to 0.07 [ppb].The copper content is generally from 0.005 to 0.1 [ppb], preferably from0.01 to 0.09 [ppb], particularly preferably from 0.015 to 0.07 [ppb].The manganese content is generally from 0.005 to 0.1 [ppb], preferablyfrom 0.008 to 0.09 [ppb], particularly preferably from 0.01 to 0.07[ppb]. The aluminum content is generally from 0.005 to 0.1 [ppb],preferably from 0.01 to 0.09 [ppb], particularly preferably from 0.02 to0.08 [ppb].

Suitable diluents for adjusting the concentration of the nitric acid,crude nitric acid or mixtures thereof to from 85 to 95% by weightinclude water, which is preferably used in the deionized form, and alsodilute nitric acid, crude nitric acid or mixtures thereof, generally5-95% strength by weight, preferably 50-95% strength by weight,particularly preferably 60-95% strength by weight, nitric acid, crudenitric acid or mixtures thereof, aqueous inorganic solutions such asdilute salt solutions, aqueous organic solutions such as alcohols oralkalis. It is likewise possible to mix highly concentrated nitric acid,crude nitric acid or a mixture thereof with water-containing solutionsin the column, instead of mixing them in the feed stream.

The entire column and its associated apparatuses above the bottom of thecolumn generally consist of nonmetallic components, preferably of glassor heat-resistant plastic such as highly fluorinated materials, forexample Teflon, which is preferably employed as lining. Particularpreference is given to using borosilicate glass. The vaporizer cancontain heating rods of, for example, tantalum.

The column preferably consists of two or more, e.g. from two to twelve,preferably from three to ten, particularly preferably from four to nine,in particular from five to eight, sections. Each of the lower sectionsis generally provided with a support grating which supports the columnpacking. The packing of the column can consist either of a bed ofpacking elements or of a structured packing made of glass, Teflon ormixtures thereof. Such columns generally have from one or two to twelve,preferably from three to ten, particularly preferably from four to nine,in particular from five to eight, theoretical plates. Suitable packingelements are all known packing elements or mixtures thereof, for exampleRaschig rings, Pall rings, Berl saddles or Prym rings.

Above the column, a condenser, e.g. a shell-and-tube heat exchangerwhich is made of glass and is operated using cooling water or, ifdesired, another coolant, can cool the high-purity nitric acid preparedaccording to the present invention. Part of the purified product can bereturned to the column.

EXAMPLES Example 1

High-concentration, 95-100% strength by weight crude nitric acid wasplaced in the bottom of a column which was made of borosilicate glass,had a ratio of column height to column diameter of 3:1, consisted of 6glass sections packed with glass Raschig rings (diameter: 25 mm) on asupport grating, was provided with a vaporizer fitted with tantalumheating rods and had an inlet for the feed between the 2nd and 3rd glasssection, and 75-95% strength by weight crude nitric acid was introducedthrough the feed inlet so as to give a nitric acid concentration of from85 to 95% by weight. A reflux ratio of 0.4 and a concentration of nitricacid taken off at the top of from 98.5 to 99.5% by weight were set. Thetemperature at the bottom was 120-125° C. and the temperature at the topwas 82 to 84° C. A high-purity nitric acid meeting the followingspecification was obtained: Cation Concentration [ppb] Sodium 0.18Chromium <0.01 Calcium 0.04 Potassium 0.03 Iron 0.04 Magnesium <0.01Zinc 0.03 Copper 0.02 Manganese 0.02 Aluminum 0.06 Anion [mg/kg] Sulfate<0.05 Chloride <0.05

1. A process for preparing high-purity nitric acid by distillation orrectification of nitric acid, crude nitric acid or mixtures thereof in acolumn, wherein from 85 to 95% strength by weight nitric acid, crudenitric acid or a mixture thereof is distilled at from 70 to 115° C. anda pressure of from 0.1 to 2 bar and nonmetallic components are used ascolumn material.
 2. A process for preparing high-purity nitric acid asclaimed in claim 1, wherein the distillation or rectification column isoperated at atmospheric pressure.
 3. A process for preparing high-puritynitric acid as claimed in claim 1, wherein the distillation orrectification column contains at least three theoretical plates.
 4. Aprocess for preparing high-purity nitric acid as claimed in claim 1,wherein the distillation or rectification column contains at least fourtheoretical plates.
 5. A process for preparing high-purity nitric acidas claimed in claim 1, wherein from 85 to 95% strength by weight nitricacid, crude nitric acid or a mixture thereof is placed in the bottom ofthe column or fed in at any point of the column, with the proviso thatfrom one to five theoretical plates are located above the feed point. 6.A process for preparing high-purity nitric acid as claimed in claim 1,wherein highly concentrated nitric acid, crude nitric acid or a mixturethereof is placed in the bottom of the column and the concentration isadjusted to from 85 to 95% by weight by means of a diluent introducedvia a feed stream and this 85-95% strength by weight nitric acid, crudenitric acid or a mixture thereof is subsequently passed through from oneto five theoretical plates.
 7. A process for preparing high-puritynitric acid as claimed in claim 1, wherein highly concentrated nitricacid, crude nitric acid or a mixture thereof is placed in the bottom ofthe column and the concentration is adjusted to from 85 to 95% by weightby means of a diluent introduced via a feed stream and this 85-95%strength by weight nitric acid, crude nitric acid or a mixture thereofis subsequently passed through from one to three theoretical plates. 8.A process for preparing high-purity nitric acid as claimed in claim 6,wherein the diluent used is water, dilute nitric acid, an aqueousinorganic or organic solution or a mixture thereof.
 9. A process forpreparing high-purity nitric acid as claimed in claim 1, wherein thefeed stream is heated to from 40 to 90° C.